theorem Th11:
  dom f in W & rng f c= W implies rng f in W
proof
  assume dom f in W;
  then rng f = f.:(dom f) & card dom f in card W by CLASSES2:1,RELAT_1:113;
  then card rng f in card W by CARD_1:67,ORDINAL1:12;
  hence thesis by CLASSES1:1;
end;
